if they just wanted to minimise resistive losses, wouldn't keeping the voltage at 18.5V (or 20V as some other laptops do) already keep the current to a minimum?
What they wanted is for the newer AC adapters to supply a higher voltage for new laptops, but still work with older models too. 18.5V is too much for the older laptops to handle.
Sorry, but I have pretty definitive evidence that you're wrong. All of the Macbook family were designed with 18.5V input. Here's the Macbook Air (A1304/M96) mentioned above, which is supplied with a 14.xV adapter:
Why would 18.5V be "too much" anyway? The input is only directly connected to buck converters and filter caps. The former are usually rated for much higher voltage. E.g. the A1304 has only an ISL6258 (appears to be some proprietary Apple part but the '57 is similar) with a 25V max input voltage, and an LT3470A - 40V(!) max input. The filter caps are 25V. Thus, you should be fine with running a Macbook from as high as 20V (I disclaim all responsibility, etc...)
(Some smaller laptops with 9-12V adapters will have 16V caps instead, and the safe input voltage on those is more like 9-13V. The next common value is 25V so that's what created the two ranges.)